Current sandwich structures used for aircraft flooring use only a single density honeycomb core in this construction. Heavy traffic on such flooring frequently causes failure or damage to the upper surface requiring that the flooring be replaced at great cost and inconvenience.
In order to increase the durability of such flooring panels and to increase their resistance to concentrated transverse compressive loads, either more skin material or higher honeycomb core densities or both must be used. Both of these remedies are basically unacceptable since they each substantially increase the panel weight. This is an unacceptable solution for aircraft structures where a minimum weight is always desired.
Many attempts to solve the problems stated above have been made. U.S. Pat. No. 4,937,125 describes one approach where a multilayer sandwich structure having a core interposed between and external and internal skin. The upper skin is a composite layer-polyester layer-honeycomb layer-metal sheet-glass fiber reinforced epoxy layer-composite layer. The core is a honeycomb. The internal skin is a metal sheet or a skin of the same composition as the external skin.
U.S. Pat. Nos. 4,336,292; 5,041,323 and 5,106,668 describe multi-layered panels comprised substantially of two or several subpanels--one possibly with a higher density core than the other(s)--bonded together to form a complete panel. The latter example would work successfully, but, because of the method and materials of construction, additional materials over and above those of the instant invention are required resulting in a panel which is too costly and substantially overweight for aerospace applications.
In the present invention, two cores of either the same or different densities and/or cell sizes are combined by means of a light weight septum to form a hybrid core. The septum material is not of the same material as the outer skins of the sandwich panel and serves as a means to transfer load between the upper and lower portions of the structure. There is no requirement that the two honeycomb cores be aligned. That the septum prevents relative movement between the two cores is necessary, but not sufficient. The septum must be both sufficiently stiff and strong to redistribute localized compressive loads as well as being sufficiently light in weight to provide panels which can meet desired weight limits.
Accordingly, improving the upper surface properties of sandwich structures which are subject to in-use concentrated loads to make them more durable is highly desired. This will reduce life-cycle costs for airlines by decreasing the necessity for replacing damaged panels. The instant invention provides that improved panel by tailoring the core to have enhanced properties just where needed (fight below the upper skin) to achieve greater resistance to concentrated transverse compressive loads with a much smaller or even no weight penalty.
Honeycomb sandwich structures are widely used for aircraft flooring materials. In these structures, the in-plane and bending properties needed are those derived from design requirements of the airframe manufacturers. The out-of-plane properties are, by contrast, determined from test data on existing materials which are believed to meet the perceived requirements. It is known that the damage that causes panels to be replaced after use in aircraft occurs almost exclusively at the top surface of the panels and is due to concentrated out-of-plane loads. This damage is believed to be due to passenger foot traffic (most likely from high heels) administering localized concentrated loads on the panels.
The instant invention addresses the issue of floor panel durability by providing constructions which increase the resistance of aircraft flooring to concentrated out-of-plane loads without either increasing weight or significantly influencing cost when compared to standard uniform density honeycomb core construction. By using higher density materials near the top (loaded) surface of a sandwich structure, the resistance to damage such as core fracture, resin fracture, core buckling, etc. due to localized compressive loads is increased.